1. Field of the Invention
The invention relates to an operation circuit for a discharge lamp which is used to operate a discharge lamp, especially a high radiance discharge lamp, such as a high pressure mercury lamp, a metal halide lamp, a xenon lamp or the like. The invention furthermore relates to a device for operating a discharge lamp using this operation circuit for a discharge lamp and a light source device using this device for operating a discharge lamp.
2. Description of the Prior Art
In a light source device for an optical device for display of images or for similar purposes, such as for example a liquid crystal projector or a DLP™ projector, a high radiance discharge lamp (HID lamp) is used. Of a light source device for such an optical device, there is a demand for prolonging the service life of the lamp and for reducing the size of the device for operating the discharge lamp.
In the case of operation of such a lamp, in the state in which a voltage which is called the no-load voltage is applied to the lamp, a high voltage is applied and thus in the discharge space an insulation breakdown is produced, and a transition after the glow discharge into an arc discharge takes place. As a process for applying a high voltage to the lamp, besides the process of superimposing a high voltage on the electrodes for the main discharge using an ignitor, i.e. besides the series trigger method, there is a process of an outside trigger method in which there is an auxiliary electrode in addition to the electrodes for the main discharge without contact with the discharge space and a high voltage is applied to this auxiliary electrode.
The outside trigger method has various advantages which the series triggering method does not, such as is described for example in Japanese patent disclosure document 2002-270386 corresponds to U.S. Pat. No. 6,552,502 or in Japanese patent disclosure document 2003-017283 corresponds to U.S. Pat. No. 6,734,643. In particular, in the case in which a high voltage generation part which comprises a high voltage transformer is separated from the feed switching part and is located in the vicinity of the discharge lamp (Ld), these advantages can be fully exploited. These advantages are: a reduction in the size and weight of the device for operating the discharge lamp, reduction of noise, increased reliability, cost reduction and the like.
The outside trigger method can be used in the same way both for a direct current operation type in which a DC voltage is applied to the two poles for the main discharge and operation is carried out, and also for an alternating current operation type in which an AC voltage is applied and the lamp is operated. The exemplary embodiments for this purpose are described in the above described Japanese patent disclosure documents 2002-270386 corresponds to U.S. Pat. No. 6,552,502, 2003-017283 corresponds to U.S. Pat. No. 6,734,643, and in Japanese patent disclosure document 2003-092198 corresponds to U.S. Pat. No. 6,661,184.
In particular, in the above described Japanese patent disclosure document 2002-270386 corresponds to U.S. Pat. No. 6,552,502 for a light source device of the direct current operation type, an outer lead pin (21A) on the cathode side of a discharge lamp (Ld′) in FIGS. 6(a) and (b) (FIGS. 6(a) and (b) are essentially identical to FIG. 9 in Japanese patent disclosure document 2002-270386 (U.S. Pat. No. 6,552,502)) is connected to the terminal (F2) of the ground of a starter (Ue) and to the terminal (T12) of the ground output of a feed circuit (Bx). Furthermore, an outer lead pin (21B) is connected on the anode side to a terminal (T11) of the plus output of the feed circuit (Bx), while an auxiliary electrode (Et) which consists of conductors (Et1, Et2, Et3) and conductive wires (W1, W2, We) is connected via the conductive wire (We) to one terminal (F3) of the output of the starter (Ye). The auxiliary electrode (Et) is connected via the conductive wire (W2) to the conductor (Et3) which is located surrounding the cathode side of a hermetically sealed portion (13). The conductor (Et3) which is located surrounding the cathode side of the hermetically sealed portion (13) is implemented by a conductive coil with which the cathode side of the hermetically sealed portion (13) is wound. Since the terminal (F3) of the output of the starter (Ue) and the terminal (F2) of the ground are connected to the two ends of a secondary winding (Se) of a high voltage transformer (Te), during the interval during which the starter (Ue) is not operated, especially during operation after completion of lamp starting, no voltage arises between the terminal (F3) of the output of the starter (Uc) and the terminal (F2) of the ground. An outer lead pin (21A) on the cathode side is connected via a terminal point (Fz) to the terminal (F2) of the ground of the starter (Ue). A state with the same electrical potential is maintained by the arrangement in which the conductor (Et3) which is located surrounding the cathode side of the hermetically sealed portion (13) is connected via the conductive wire (W2), the auxiliary electrode (Et) and the conductive wire (We), during operation in line installation from the outer lead pin (21A) of the cathode side to the terminal (T12) of the ground output of the feed circuit (Bx) at the terminal point (Fz) of the terminal (F2) of the ground of the starter (Ue) and on the conductor (Et3) which is arranged such that it surrounds the cathode side of the hermetically sealed portion (13). In the operating state of the discharge lamp (Ld′) the main discharge current of the discharge lamp (Ld′) flows in the line path from the tip of a cathode (14) via the outer lead pin (21A) of the cathode side as far as the terminal point (Fz), by which a voltage reduction is formed which is proportional to the product of the resistance value of this line path and the value of the flowing current. The closer the tip of the cathode (14) is approached, the higher the electrical potential becomes.
As was described above, the terminal point (Fz) and the conductor (Et3) have the same electrical potential. The cathode, especially the vicinity of the hermetically sealed portion (13), therefore has a higher electrical potential than the conductor (Et3) which surrounds its periphery. As is described in Japanese patent HEI 4-40828 corresponds to U.S. Pat. No. 4,673,843, as a result in the discharge vessel (11) of the lamp which has reached a high temperature in operation, the metallic cations of the impurities which are contained in the material of the discharge vessel (11) in the vicinity of the hermetically sealed portion (13) thereof are driven in a direction which moves away from the electrode material comprising the cathode. It is furthermore described: Since the phenomenon of detachment of the glass material such as silica glass or the like of the hermetically sealed portion of the discharge vessel from the electrode material as a result of accumulation of metallic cations of impurities on the surface of the electrode material is prevented, the lamp arrangement described above using FIGS. 6(a) and (b) yields the action of preventing the disadvantage of lamp damage which is caused by the above described detachment phenomenon.
However, if an attempt is made to implement the light source device shown in FIG. 7, in which the above described arrangement as shown in FIG. 6 is used for alternating current type of operation (the circuit arrangement in FIG. 7 is essentially identical to FIG. 6 in Japanese patent disclosure document 2002-270386 corresponds to U.S. Pat. No. 6,552,502), for each half period of the AC voltage a polarity inversion of the voltage applied to the discharge lamp (Ld) takes place. In the half period phase in which the above described auxiliary electrode (Et) reaches a cathode-side electrical potential, the disadvantage of lamp damage as a result of the phenomenon of detachment of the glass material such a silica glass or the like of the hermetically sealed portion of the discharge vessel from the above described electrode material is prevented. In the half period phase in which the above described auxiliary electrode (Et) reaches the anode-side electrical potential, this disadvantage is not prevented. Furthermore, in the case of a low alternating driving frequency for application to a discharge lamp (Ld) of for example less than or equal to 3.5 kHz, there is the possibility that in the half period phase in which the above described auxiliary electrode (Et) reaches the cathode-side electrical potential, especially in the hermetically sealed portion of the discharge vessel which in this half period phase represents the cathode side, this disadvantage is exacerbated. This circumstance constituted an unresolved task with respect to implementation of such a light source device.
Compared to the arrangement in which the conductor (Et3) of the auxiliary electrode (Et) of the discharge lamp (Ld′) described above using FIGS. 6(a) and (b) actively surrounds the hermetically sealed portion of the discharge vessel, in the discharge lamp (Ld) described in FIG. 7 the hermetically sealed portion of the discharge vessel is not actively surrounded by the auxiliary electrode (Et). When the auxiliary electrode (Et) is located in the vicinity of the hermetically sealed portion of the discharge vessel, it is possible for the above described disadvantage to be exacerbated as before.